12 books • 4 series
Book #1: Pocket Sudoku Hard
Book #1: Sudoku Puzzles Hard
Book #1: Sudoku Puzzles Medium
Book #1: 1,000+ Sudoku Puzzles Easy to Hard
Book #2: Sudoku Puzzles Easy
Book #1: Sudoku 2000
Book #1: Sudoku Puzzles Inhuman